During the 2008 Olympics, there was hope. The prospect of China’s coming-out party being swathed in a choking haze prompted Beijing to take major steps to clean up its notoriously bad air: moving factories out of the city, reducing coal-fired boilers and home heaters, and raising vehicle-emission standards. And by the time the Olympic party was rolling, Beijing’s air turned miraculously clear, helped in part by emergency measures like taking half the city’s cars off the roads. There was hope after the Olympics too. In the first half of 2009 the Chinese capital enjoyed its best air quality in almost a decade.

Since then, positive signs have been harder to find. Take Friday, when an early season flurry fell amid bad air, producing a surreal snow falling on smog effect. And yesterday, when noxious emissions combined with a cold fog that grounded hundreds of flights at the Beijing Capital International Airport. The Beijing Environmental Protection Bureau (EPB) says that air quality in the city has consistently improved in recent years. But medical experts quoted by the state-run China Daily said that lung cancer has climbed 60% over the past decade, even the number of smokers has remained steady, and air pollution may be to blame.

If one wants to find a hopeful sign about Beijing’s air quality, it is best not to look to the skies. Rather, the most positive developments can be found online. On the Chinese Twitter-like service Sina Weibo, users have begun to post results from the air-quality monitor operated by the U.S. embassy in Beijing. The embassy publishes hourly results on Twitter, which is blocked in China, but those tweets can easily be transferred to Weibo via an iPhone application. The embassy’s pollution data has generated concern from the Chinese government. In June 2009, after TIME reported on the air-quality monitor, the Chinese Foreign Ministry complained about the embassy’s public reporting of pollution data, according to this WikiLeaks cable. The Chinese officials said that by posting the data online, the U.S. was causing confusion among the Chinese public that might lead to “social consequences.” They asked that the U.S. consider ways to limit the distribution of the data. At that time Twitter was not permanently blocked in China. It was temporarily censored in early June 2009 during the 20th anniversary of the Tiananmen crackdown, then it was made accessible again later that month. In July 2009 the Chinese authorities ended up restricting the publication of the air-quality information themselves when they blocked Twitter permanently during the deadly race riots in the northwestern city of Urumqi. The embassy didn’t curtail reporting of the data, and has in fact taken steps to make it more widely available, posting it on a Web page that is currently not blocked in China.

In recent months many Weibo users, including prominent real estate developer Pan Shiyi, whose microblog has nearly 8 million followers, have been raising questions about the discrepancies between the air-quality numbers posted by the Beijing EPB and those of the U.S. embassy. On Monday, for instance, while the @beijingair feed posted several hours of ratings that were deemed “hazardous” according to U.S. standards, Beijing’s official reading for the day was “lightly polluted.” There are several reasons for the differences. The embassy monitors from a single site that tests for tiny particles known as PM 2.5 that are particularly hazardous when inhaled. The government uses readings for larger particles known as PM 10, as well as sulfur dioxide and nitrogen dioxide, that are taken by 27 stations around the city. Its figure, posted each afternoon, describes the air quality for the previous 24 hours. The Beijing EPB has criticized the U.S. data, saying that it’s too limited to provide a complete picture. “It is not scientific to evaluate a city’s air quality from a single spot,” Yu Jianhua, director of the Beijing EPB’s air-pollution-control division, said last month. “It’s our current standard, which is in line with the international standard, to evaluate [air quality] based on 24-hour average levels. It’s completely meaningless to compare our results with those based on single-hour levels.”

And yet many in Beijing rely on the hourly U.S. embassy reports. Yesterday’s 24-hour average is of little use when trying to decide if you want to go for a walk this afternoon. The China National Environmental Monitoring Center did launch hourly reporting online one year ago, but the website is so difficult to use that it is largely worthless. On Weibo, Pan and others are arguing for PM 2.5 reporting in Beijing. Yu told me that Beijing has already been monitoring the tiny particles, but has held off reporting as a national standard has not yet been set. On Sunday the English edition of the Communist Party-run Global Timescited unnamed experts who suggested that when PM 2.5 is added to the national air-pollution index, “more than 70 percent of China’s cities would fall below the good-air-quality bar, which would translate into poor performance readings for officials.” When I raised that issue with Yu, he denied that was a concern. But PM 2.5, which in addition to vehicle exhaust is produced by the reaction between gases in the air, is notoriously difficult to control. Research conducted by the Chinese Academy of Sciences over the past decade found that while PM 10 was decreasing 2% to 3% annually, PM 2.5 was going up 3% to 4% each year, the China Daily said last month. After the city of Nanjing posted a PM 2.5 reading on Sina Weibo last month that was nearly four times the standard recommended by the World Health Organization, it quickly deleted the message, the Global Timesreported.

Once the PM 2.5 is incorporated into the air-pollution index, the picture might not be that much clearer. Environmental consultant Steven Q. Andrews, who in 2008 raised serious questions about whether Beijing officials were manipulating air data ahead of the Olympics, published a piece on Monday on chinadialogue.net, a website run by an environmental nonprofit, examining the U.S. embassy data and the proposed PM 2.5 standard. He argues that the severity of Beijing’s air quality problems will continue to be underplayed once PM 2.5 is officially included.

Now environmentalists in Beijing are beginning to check PM 2.5 on their own. Amid the hazy outlook for Beijing’s air quality, that alone counts as a sign of hope.

Austin Ramzy is Beijing correspondent for TIME. Find him on Twitter at @austinramzy. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.